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Fuji-Hakone-Izu NP, Shizuoka
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A 2022 study overturned the old idea that Hoeizan’s rusty “Akaiwa” is ancient rock pushed up by magma: its rock chemistry and magnetic record show it formed from deposits of the 1707 eruption.
In Hokusai’s print “Appearance of Hoeizan,” the new mountain is nowhere in sight. He evokes its emergence through collapsing houses and billowing dust, turning the eruption into a human-scale scene.
The 1707 eruption buried the old Suyama pilgrimage trail and huts. The route later shifted east of Hoeizan; after years of village upkeep, 5,398 pilgrims climbed from Suyama in 1800, including 18 women.
Hoeizan stands beside 3 craters, but the 1707 eruption’s activity shifted from the lower southeast craters, No. 2 and No. 3, toward No. 1, higher up and nearer Fuji’s summit.
This was no lava-flow eruption: it began with pale pumice, then shifted to dark scoria as the magma’s composition changed. The material fell as pumice, scoria, and ash rather than flowing as lava.
Hoei No. 1 crater is wider than Fuji’s summit crater. Its upper wall exposes stacked old lava flows sliced through by vertical dikes—the frozen pathways magma once took through the mountain.
Ash from the 1707 eruption reached Edo, today’s Tokyo. Writer Arai Hakuseki described the daytime gloom as so deep that people lit lanterns indoors.
A 1712 illustrated encyclopedia—just 5 years after the eruption—already describes a small hill appearing beside Fuji and being called Hoeizan, preserving an early account of the mountain’s name.
The floor of Hoei No. 1 holds a partly collapsed, pancake-shaped spatter cone, thought to have formed near the eruption’s end: sticky, still-molten fragments splashed down and piled together.
Among the volcanic bombs and scoria scattered across the crater floor are fragments of gabbro—a rock that solidified deep inside Fuji’s magma chamber before the eruption carried it upward.
The Hoei eruption began on December 16, 1707, exactly 49 days after the devastating Hoei earthquake of October 28. It is a striking sequence in the historical record.
